Countercurrent mass transfer methods, such as distillation or absorption, are normally carried out in apparatus employing liquid gravity downflow or crossflow in contact with an upward flow of gas. Conventionally, such contact is carried out either stagewise, employing a variety of liquid-gas contacting trays, or in so-called differential contacting equipment, such as packed towers.
A large variety of contacting trays are known to the art, mainly comprising bubble cap or sieve trays, wherein liquid flows horizontally across each contact tray in crossflow relation to the gas flowing generally vertically upward through the tray. In such conventional contacting trays, intimate gas-liquid contact is secured by bubbling the gas through the liquid on the tray and generating a bubble froth.
On conventional bubble-cap and sieve trays, liquid droplet spray or entrainment is known to be incidentally generated. Such spray carryover is known in the art as "liquid-in-gas" entrainment. This entrainment is considered to be undesirable in stage-wise, countercurrent, liquid-gas contact, because spray carryover of liquid from one tray to the contact tray above short-circuits the desired progression of tray-to-tray liquid/gas concentration gradients. The prior art has attempted to solve this problem in various ways. For example, Kiselev, in U.S. Pat. No. 4,820,456, uses perforated plate froth retainer cells to generate a fine foam in order to blanket the turbulent liquid and inhibit entrainment carryover to the tray above.
In some mass transfer methods, where a significant portion of the total transfer resistance lies in the liquid phase, spray contacting may be desirable. Examples of such processes include the de-aeration of boiler feed water, stripping of volatile organic contaminants from contaminated water and absorption of highly-soluble gases. These, and other similar processes, are characterized by the fact that the overall transfer rate is controlled by liquid-phase or liquid-film diffusion rates, which are intrinsically slower than gas-phase diffusion rates. The minimization of such resistance requires the maximum degree of continuous mixing of the liquid phase. However, spray contactors do not normally provide for optimum or continuous mixing of the liquid phase, and therefore have not been widely used industrially for gas-liquid mass transfer operations.
In a spray contactor, the major fraction of the total mass transfer of a solute from a gas to a liquid, or from the liquid to the gas, occurs during drop formation in the vicinity of the spray nozzles. During drop formation, the liquid is in generally turbulent sheet or jet flow. Once the liquid drops are formed, they mix internally by oscillation for a brief period and subsequently behave more or less like rigid spheres, with no further internal mixing. Within a liquid drop, liquid phase transfer in the absence of internal mixing is a slow diffusion-controlled process which yields very low overall transfer rates. This absence of internal stirring or mixing following drop formation is one of the major disadvantages of conventional single-stage spray contact devices.
One method of achieving liquid mixing following the formation of liquid drops is to collect, or coalesce, the drops into bulk liquid form and then reform the drops. Prior art methods for drop collection and spray regeneration have generally involved some external mechanical device, such as a recycle pump and spray nozzles. Mechanical pumping to secure spray regeneration is energy-intensive and is not economical where a number of contact stages is required.
Such prior art spray contactors that rely on mechanical means of controlled spray liquid capture and recycle include Herrlander, U.S. Pat. No. 4,514,196, which utilizes a contact tray containing a plurality of separate venturi tubes with a spider-arm liquid distributor, with one liquid tube feeding each individual venturi. The gas venturis generate upward liquid spray. The liquid spray is intercepted by a bed of balls where secondary bubble flow contacting is secured. There is no internal recycle of liquid; passage of liquid is essentially once-through. In another example, Ekman, U.S. Pat. No. 3,795,486, teaches the use of a series of spaced-apart cylindrical rods with spray injected either from above or below into vertical gas upflow. The flow of liquid is "downwardly counter-current" to gas flow through the vessel containing the series of spaced-apart rods. The combination of rod spacing and gas velocities used by Ekman yields downward cascading of liquid through the rod arrays and generally countercurrent liquid/gas flow. Andersen, U.S. Pat. No. 3,447,287, effects once-through passage of smoke and spray through an array of rods in order to obtain particulate collection. In Ekman, U.S. Pat. No, 4,140,501, gas flows horizontally through a single row of spaced-apart vertical or inclined venturi-forming tubes. The tube array is face-sprayed by an upstream spray nozzle cocurrent with gas flow, and the spray which is entrained downstream is removed by a series of progressively finer vane demisters. Demisted liquid is drained from the system, and there is no teaching in Ekman of liquid recycle. In all of these cases of the prior art, liquid spray generation and/or collection is random and uncontrolled other than by gravity. There is no means provided for control of, or internal recycle of, the spray liquid. In neither Ekman or Andersen is there any teaching of a method or means for internal spray collection, directed external flow and automatic regeneration of spray recycle of liquid in a controlled, repeated, manner.
In Lerner, U.S. Pat. No. 4,732,585, spaced apart perforated or foraminous tubes, closed at the ends, are used in a baffle-and-tube array to cause liquid in the tubes to go into bubbling flow. Gas-liquid contacting is by means of bubbling flow, with spray injected above the array, and liquid draining downward from the bubble tubes. Lerner provides no method or means for internally-controlled liquid recycle and spray regeneration.